diff --git a/gen_header.go b/gen_header.go
index 6d4f58d..54423e1 100644
--- a/gen_header.go
+++ b/gen_header.go
@@ -9,6 +9,7 @@ import (
 	"go/token"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 	"strings"
 )
 
@@ -25,47 +26,52 @@ func main() {
 }
 
 func run() error {
-	src, err := ioutil.ReadFile("./src/api.go")
-	if err != nil {
-		return err
-	}
-	fs := token.NewFileSet()
-	tree, err := parser.ParseFile(fs, "api.go", src, parser.ParseComments)
-	if err != nil {
-		return err
-	}
 	buf := new(bytes.Buffer)
 	buf.WriteString(`
 #ifdef __cplusplus
 extern "C" {
 #endif
 `)
-	for _, dec := range tree.Decls {
-		fnc, ok := dec.(*ast.FuncDecl)
-		if !ok {
-			continue
+	defer buf.WriteString(`
+#ifdef __cplusplus
+}
+#endif
+`)
+	for _, fname := range []string{
+		"api.go",
+		"src_index.go",
+	} {
+		src, err := ioutil.ReadFile(filepath.Join("./src", fname))
+		if err != nil {
+			return err
 		}
-		doc := fnc.Doc.Text()
-		pref := "export " + fnc.Name.Name
-		if !strings.Contains(doc, pref) {
-			continue
+		fs := token.NewFileSet()
+		tree, err := parser.ParseFile(fs, fname, src, parser.ParseComments)
+		if err != nil {
+			return err
 		}
-		buf.WriteString("\n")
-		for _, c := range fnc.Doc.List {
-			if strings.HasPrefix(c.Text, "//"+pref) {
+		for _, dec := range tree.Decls {
+			fnc, ok := dec.(*ast.FuncDecl)
+			if !ok {
 				continue
 			}
-			buf.WriteString(c.Text + "\n")
-		}
-		if err = writeCSignature(buf, fnc); err != nil {
-			return err
+			doc := fnc.Doc.Text()
+			pref := "export " + fnc.Name.Name
+			if !strings.Contains(doc, pref) {
+				continue
+			}
+			buf.WriteString("\n")
+			for _, c := range fnc.Doc.List {
+				if strings.HasPrefix(c.Text, "//"+pref) {
+					continue
+				}
+				buf.WriteString(strings.TrimRight(c.Text, "\n") + "\n")
+			}
+			if err := writeCSignature(buf, fnc); err != nil {
+				return err
+			}
 		}
 	}
-	buf.WriteString(`
-#ifdef __cplusplus
-}
-#endif
-`)
 	return writeHeader(buf.Bytes())
 }
 
diff --git a/go.mod b/go.mod
index 4f3c616..c835596 100644
--- a/go.mod
+++ b/go.mod
@@ -13,5 +13,5 @@ require (
 	gopkg.in/yaml.v2 v2.2.2 // indirect
 )
 
-// FIXME: remove once https://github.com/bblfsh/sdk/pull/403 is merged
-replace github.com/bblfsh/sdk/v3 => github.com/dennwc/sdk/v3 v3.0.0-20190603173141-0c143b25c491
+// FIXME: remove once https://github.com/bblfsh/sdk/pull/403 is released
+replace github.com/bblfsh/sdk/v3 => github.com/bblfsh/sdk/v3 v3.1.1-0.20190606133740-c75dbe779e6b
diff --git a/go.sum b/go.sum
index 6d064d4..803aec3 100644
--- a/go.sum
+++ b/go.sum
@@ -8,6 +8,8 @@ github.com/antchfx/xpath v0.0.0-20190319080838-ce1d48779e67 h1:uj4UuiIs53RhHSySI
 github.com/antchfx/xpath v0.0.0-20190319080838-ce1d48779e67/go.mod h1:Yee4kTMuNiPYJ7nSNorELQMr1J33uOpXDMByNYhvtNk=
 github.com/bblfsh/sdk/v3 v3.1.0 h1:V+4DqGlqbxiAEAU0XlpTjjbq/g0ljfiFoh9jsIkfbtU=
 github.com/bblfsh/sdk/v3 v3.1.0/go.mod h1:juMiu8rP3lYJN1e4neEkSyzNieqiFceZzN4AOo0Rm1Q=
+github.com/bblfsh/sdk/v3 v3.1.1-0.20190606133740-c75dbe779e6b h1:61YtAI8BRF+tWMFlgmuWujS1zzGf5HY76QkMsDBJsMw=
+github.com/bblfsh/sdk/v3 v3.1.1-0.20190606133740-c75dbe779e6b/go.mod h1:juMiu8rP3lYJN1e4neEkSyzNieqiFceZzN4AOo0Rm1Q=
 github.com/codahale/hdrhistogram v0.0.0-20161010025455-3a0bb77429bd/go.mod h1:sE/e/2PUdi/liOCUjSTXgM1o87ZssimdTWN964YiIeI=
 github.com/containerd/continuity v0.0.0-20180712174259-0377f7d76720/go.mod h1:GL3xCUCBDV3CZiTSEKksMWbLE66hEyuu9qyDOOqM47Y=
 github.com/davecgh/go-spew v1.1.0 h1:ZDRjVQ15GmhC3fiQ8ni8+OwkZQO4DARzQgrnXU1Liz8=
diff --git a/src/src_index.go b/src/src_index.go
new file mode 100644
index 0000000..0a76a74
--- /dev/null
+++ b/src/src_index.go
@@ -0,0 +1,180 @@
+package main
+
+/*
+#include "uast.h"
+#include <stdlib.h>
+*/
+import "C"
+
+import (
+	"sync"
+	"unsafe"
+
+	"github.com/bblfsh/sdk/v3/uast/transformer/positioner"
+)
+
+var (
+	srcMu      sync.RWMutex
+	srcLast    Handle
+	srcIndexes = make(map[Handle]*positioner.Index)
+)
+
+// newSourceIndex allocates and populates the UastSourceIndex structure for C code to use.
+func newSourceIndex(h Handle) *C.UastSourceIndex {
+	sz := unsafe.Sizeof(C.UastSourceIndex{})
+	v := (*C.UastSourceIndex)(C.malloc(C.size_t(sz)))
+	v.handle = C.uintptr_t(h)
+	return v
+}
+
+//export UastSourceIndexNew
+// UastSourceIndexNew creates a positional index for a source file. An index can be used
+// to convert byte-based node offsets to Unicode character offsets.
+func UastSourceIndexNew(source unsafe.Pointer, size C.size_t) *C.UastSourceIndex {
+	index := positioner.NewIndex(cBytes(source, size), &positioner.IndexOptions{
+		Unicode: true,
+	})
+	srcMu.Lock()
+	srcLast++
+	h := srcLast
+	srcIndexes[h] = index
+	srcMu.Unlock()
+	return newSourceIndex(h)
+}
+
+//export UastSourceIndexFree
+// UastSourceIndexFree release all resources associated with an index.
+func UastSourceIndexFree(idx *C.UastSourceIndex) {
+	if idx == nil {
+		return
+	}
+	h := idx.handle
+	C.free(unsafe.Pointer(idx))
+	if h == 0 {
+		return
+	}
+	srcMu.Lock()
+	delete(srcIndexes, Handle(h))
+	srcMu.Unlock()
+}
+
+// getSourceIndex returns an index for a given handle. It returns nil if handle is invalid.
+func getSourceIndex(h Handle) *positioner.Index {
+	srcMu.RLock()
+	idx := srcIndexes[h]
+	srcMu.RUnlock()
+	return idx
+}
+
+// convertOffset is a helper to convert one offset to the other, given a conversion function.
+func convertOffset(fnc func(*positioner.Index, int) (int, error),
+	idx *C.UastSourceIndex, off C.int) C.int {
+	if idx == nil || idx.handle == 0 || off < 0 {
+		return -1
+	}
+	index := getSourceIndex(Handle(idx.handle))
+	if index == nil {
+		return -1
+	}
+	roff, err := fnc(index, int(off))
+	if err != nil {
+		// TODO: we should return this error somehow
+		//       previously we stored it in Context, but we don't have (nor need) it here
+		return -1
+	}
+	return C.int(roff)
+}
+
+// convertToLineCol is a helper to convert one offset to the line-column pair, given a conversion function.
+func convertToLineCol(fnc func(*positioner.Index, int) (int, int, error),
+	idx *C.UastSourceIndex, off C.int) C.UastLineCol {
+	if idx == nil || idx.handle == 0 || off < 0 {
+		return C.UastLineCol{-1, -1}
+	}
+	index := getSourceIndex(Handle(idx.handle))
+	if index == nil {
+		return C.UastLineCol{-1, -1}
+	}
+	line, col, err := fnc(index, int(off))
+	if err != nil {
+		// TODO: we should return this error somehow
+		//       previously we stored it in Context, but we don't have (nor need) it here
+		return C.UastLineCol{-1, -1}
+	}
+	var pos C.UastLineCol
+	pos.line = C.int(line)
+	pos.col = C.int(col)
+	return pos
+}
+
+//export UastSourceIndexFromLineCol
+// UastSourceIndexFromLineCol converts one-based line-column pair (in bytes) in the indexed
+// source file to a zero-based byte offset. It return -1 in case of failure.
+func UastSourceIndexFromLineCol(idx *C.UastSourceIndex, line, col C.int) C.int {
+	if idx == nil || idx.handle == 0 || line < 0 || col < 0 {
+		return -1
+	}
+	index := getSourceIndex(Handle(idx.handle))
+	if index == nil {
+		return -1
+	}
+	off, err := index.Offset(int(line), int(col))
+	if err != nil {
+		// TODO: we should return this error somehow
+		//       previously we stored it in Context, but we don't have (nor need) it here
+		return -1
+	}
+	return C.int(off)
+}
+
+//export UastSourceIndexToLineCol
+// UastSourceIndexToLineCol converts zero-based byte offset in the indexed source
+// file to a one-based line and one-based column pair (in bytes).
+// It return a UastLineCol with both elements set to -1 in case of failure.
+func UastSourceIndexToLineCol(idx *C.UastSourceIndex, off C.int) C.UastLineCol {
+	return convertToLineCol((*positioner.Index).LineCol, idx, off)
+}
+
+//export UastSourceIndexFromUnicode
+// UastSourceIndexFromUnicode converts zero-based Unicode character offset in the indexed
+// source file to a zero-based byte offset. It return -1 in case of failure.
+func UastSourceIndexFromUnicode(idx *C.UastSourceIndex, off C.int) C.int {
+	return convertOffset((*positioner.Index).FromRuneOffset, idx, off)
+}
+
+//export UastSourceIndexToUnicode
+// UastSourceIndexToUnicode converts zero-based byte offset in the indexed source file to
+// a zero-based Unicode character offset. It return -1 in case of failure.
+func UastSourceIndexToUnicode(idx *C.UastSourceIndex, off C.int) C.int {
+	return convertOffset((*positioner.Index).ToRuneOffset, idx, off)
+}
+
+//export UastSourceIndexFromUTF16
+// UastSourceIndexFromUTF16 converts zero-based UTF-16 code point offset in the indexed
+// source file to a zero-based byte offset. It return -1 in case of failure.
+func UastSourceIndexFromUTF16(idx *C.UastSourceIndex, off C.int) C.int {
+	return convertOffset((*positioner.Index).FromUTF16Offset, idx, off)
+}
+
+//export UastSourceIndexToUTF16
+// UastSourceIndexToUTF16 converts zero-based byte offset in the indexed source file to
+// a zero-based UTF-16 code point offset. It return -1 in case of failure.
+func UastSourceIndexToUTF16(idx *C.UastSourceIndex, off C.int) C.int {
+	return convertOffset((*positioner.Index).ToUTF16Offset, idx, off)
+}
+
+//export UastSourceIndexToUnicodeLineCol
+// UastSourceIndexToUnicodeLineCol converts zero-based byte offset in the indexed source
+// file to a one-based line and one-based column pair (in Unicode characters).
+// It return a UastLineCol with both elements set to -1 in case of failure.
+func UastSourceIndexToUnicodeLineCol(idx *C.UastSourceIndex, off C.int) C.UastLineCol {
+	return convertToLineCol((*positioner.Index).ToUnicodeLineCol, idx, off)
+}
+
+//export UastSourceIndexToUTF16LineCol
+// UastSourceIndexToUTF16LineCol converts zero-based byte offset in the indexed source
+// file to a one-based line and one-based column pair (in UTF-16 code units).
+// It return a UastLineCol with both elements set to -1 in case of failure.
+func UastSourceIndexToUTF16LineCol(idx *C.UastSourceIndex, off C.int) C.UastLineCol {
+	return convertToLineCol((*positioner.Index).ToUTF16LineCol, idx, off)
+}
diff --git a/src/uast.h b/src/uast.h
index 6471dd3..37578ed 100644
--- a/src/uast.h
+++ b/src/uast.h
@@ -149,6 +149,18 @@ typedef struct UastMemStats {
  uint64_t objects;   // number of live objects
 } UastMemStats;
 
+// UastSourceIndex represents a positional index for a source file.
+// It's initialized with UastSourceIndexNew and freed with UastSourceIndexFree.
+typedef struct UastSourceIndex {
+  uintptr_t handle;
+} UastSourceIndex;
+
+// UastLineCol is a one-based line-column position in a source file.
+typedef struct UastLineCol {
+  int line;
+  int col;
+} UastLineCol;
+
 /*GO-HEADER*/
 
 #endif // UAST_H_
\ No newline at end of file